High Density Perforating Gun System Producing Reduced Debris

ABSTRACT

A perforating system has a perforating module comprising a unitary body of explosive. The explosive is contained within a non-explosive casing, or liner, having formed indentations and a cover thereover. The indentations, which will transform into explosively formed penetrators (EFP&#39;s) upon detonation, have a perimeter shape that allows for improved packing density, e.g., a hexagonal perimeter, which results in relatively little “dead space” wherein no perforating penetrators are generated. In operation, the module provides a relatively dense shot pattern and substantially reduced amount of post-detonation debris that could clog the perforations and/or require remedial clean-up or repeat perforation.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of and claims priority from U.S. patent application Ser. No. 12/541,827 filed Aug. 14, 2009, which in turn is a continuation-in-part of and claims priority from U.S. patent application Ser. No. 11/405,148, filed on Apr. 17, 2006, all of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure relates generally to the design of perforating tools for use in creating perforations in wellbores to improve the flow of fluids from the wellbore.

2. Description of the Related Art

Commercial development of hydrocarbon fields requires significant amounts of capital. Therefore, before field development begins, operators desire to have as much information as possible in order to evaluate the reservoir for commercial viability. Such information may be acquired at the seismic exploration phase, during well construction, prior to well completion and/or any time thereafter.

Perforation guns are used within wellbore holes to increase the permeability of the formation surrounding the wellbore. In general, perforation guns producing greater numbers of perforations are considered to be more effective than those producing fewer perforations. It is therefore often desired to maximize the number of penetrating jets within a segment of the wellbore. This may be difficult, however, because there are limitations relating to placement of the charges used for perforation. Standard shaped charges have an outer housing formed of metal or another material that encloses the high explosive charge. The shaped charge holder has openings that have typically circular perimeters. When packing the charges in an adjoining manner in the charge tube, interstitial spaces are unavoidably left between the neighboring charges as a result their shape. This packing of the charges results in significant “dead spaces,” that is, areas from which no perforating product, i.e., no jets, is/are provided, between the charges, and limits the density with which the charges can be packed.

There are a number of known styles and designs for perforation guns. There are, for example, strip guns that include a strip carrier upon which are mounted a number of capsule charges. The capsule charges are individually sealed against corrosive wellbore fluids. Also known are hollow carrier guns that have a sealed outer housing that contains unencapsulated shaped charges. In each case, the shaped charges are arranged such that they will detonate in a radially outward direction to form a specific pattern of perforations.

An alternative perforation gun design is described in U.S. Pat. No. 5,619,008 to Chawla et al. In this design, a two-layer liner serves to sheath discontinuous loadings of explosive material. The liner is configured with indentations that are each aligned with an individual loading of the explosive material. Upon detonation of the loadings of explosive material, these indentations act in the manner of a shaped charge, creating a directed jet of liner material. The indentations have a circular perimeter and are spaced apart from one another, leaving significant “dead space” between them. Following detonation and any resulting perforation, the housing that surrounds the charges is not completely destroyed and forms debris. This debris is undesirable, both because it must be removed by wireline or by other means in a secondary operation, and because it may clog the perforations that are formed by the perforation operation, thereby making the perforations less effective and sometimes necessitating repeat perforation operations. The Chawla et al. invention thus suffers from problems relating to both “dead space” and debris creation.

The present disclosure addresses the problems of the prior art.

SUMMARY OF THE DISCLOSURE

The present disclosure provides a perforating device that produces multiple perforating penetrators from a single high explosive charge. In one embodiment, the perforating module has a central rod with a surrounding cylinder of high explosive. The cylinder of high explosive is contained within a liner having formed indentations. The liner may be of any suitable material, such as a non-explosive material including, for example, an elemental metal or alloy, a composite, a ceramic, a thermoplastic or thermo set polymer, or the like. Finally, a cylindrical outer cover is disposed about the liner. In one embodiment, the indentations are linearly contiguous to one another. In another embodiment, the indentations each have a perimeter that is triangular, square, hexagonal, or octagonal and are disposed in an adjoining fashion to one another.

In operation and as a result of detonation of the explosive material, the module forms penetrators of liner material that propagate into the formation in a direction that is, in one embodiment, substantially perpendicular to the longitudinal axis of the wellbore. The module thus is capable of providing a relatively dense shot pattern with little or no “dead space” between the locations from which the penetrators are formed. This results in an effective perforation of a wellbore segment.

During the detonation, the constituent components of the module, the high explosive, the liner, and the outer cover, are largely destroyed. As a result, the amount of debris resulting from the detonation is reduced or eliminated, in contrast with the large amount of debris produced by many conventional perforation devices.

One embodiment of the disclosure includes a method for perforating a subterranean formation, comprising: lowering a plurality of perforating modules in a wellbore, wherein each of the plurality of perforating modules comprises: a central rod, wherein the central rod comprises: an exterior load bearing portion, and an interior detonation portion including a first explosive, a second explosive adapted to at least partially surround the central rod, and a liner disposed to surround the second explosive, wherein the liner is made of a non-explosive material, and wherein the liner has a plurality of concave arcuate surface indentations; and detonating the plurality of perforating modules in the wellbore.

Another embodiment of the disclosure includes a method for perforating a subterranean formation, comprising: lowering a plurality of perforating modules in a wellbore, wherein at least one of the plurality of perforating modules is separated from another of the plurality of perforating modules by a spacer, and wherein each of the plurality of perforating modules comprises: a central rod, wherein the central rod comprises: an exterior load bearing portion comprised of a ceramic, an axial passage adapted to receive hydraulic fluid disposed along the length of the exterior load bearing portion, a wire disposed within the axial passage, and an interior detonation portion including a first explosive, a second explosive adapted to at least partially surround the central rod, a liner disposed to surround the second explosive, wherein the liner is made of a non-explosive material, and wherein the liner has a plurality of concave arcuate surface indentations, wherein each of the plurality of concave arcuate surface indentation is polygonal, wherein each of the plurality of concave arcuate surface indentations is configured to face substantially perpendicular to the longitudinal axis of the wellbore, and wherein each of the plurality of concave arcuate surface is linearly contiguous with at least another of the plurality of concave arcuate surface indentations, and a cover disposed about the liner; and detonating the plurality of perforating modules in the wellbore.

In embodiments, the plurality of shallow concave surface indentations may each have a cavity. The cavity may be defined by a diameter and a depth. In one arrangement, the diameter to depth ratio is approximately not less than two to one. In another embodiment, the diameter to depth ratio is not less than six to one. In still other embodiments, the depth is no greater than a thickness of the liner. The method may also include forming a plurality of perforations in a region adjacent to the perforating modules, wherein the plurality of perforations extend substantially through a cement layer into a formation a distance from a cement face that is no greater than a diameter of the cavity. In some applications, the distance is no greater than one-half of the diameter of the cavity. Also, the method may include at least partially lining the plurality of perforations in the cement layer with a liner material.

Another embodiment of the disclosure includes a method for perforating a subterranean formation. The method may include lowering a perforating module into a wellbore having a casing incased in cement, the perforating module having at least one explosively formed penetrator forming charge and liner; positioning the plurality of perforating modules in the wellbore and adjacent to a substantially unconsolidated formation; perforating the casing and cement; and perforating the formation to a distance no greater than a diameter of the at least one explosively formed penetrator forming charge and liner, wherein the distance measured from a boundary between the cement and the formation.

BRIEF DESCRIPTION OF THE DRAWINGS

For greater understanding of the disclosure, reference is made to the following detailed description of the embodiments of the present disclosure, taken in conjunction with the accompanying drawings in which reference characters designate like or similar elements throughout the several figures of the drawings.

FIG. 1 is a side, cross-sectional view of a wellbore containing an exemplary perforation system constructed in accordance with the present disclosure.

FIGS. 1 a and 1 b illustrate a pair of alternative constructions for perforation systems constructed in accordance with the present disclosure.

FIG. 2 is a side, cross-section depiction of a single perforation module of the perforation system shown in FIG. 1.

FIG. 3 is an exterior view of the module shown in FIG. 2.

FIG. 4 is a detail view of a portion of the liner of an exemplary perforation module showing further details concerning the indentations.

FIG. 5 is a detail view of a portion of the liner of an exemplary perforation module showing an alternative shape for the indentations.

FIG. 6 is a side cross-section of the portion of liner shown in FIG. 5, taken along lines 6-6.

FIG. 7 depicts an exemplary shot pattern that is created by the perforation module shown in FIGS. 2 and 3.

FIG. 8 illustrates an alternative embodiment for a perforation module in accordance with the present disclosure having triangular indentations.

FIG. 9 illustrates a further alternative embodiment for a perforation module in accordance with the present disclosure having square indentations.

FIG. 10 depicts a portion of the surface of the liner of a perforation module that utilizes octagonal indentations.

FIGS. 11-15 illustrate an exemplary initiation sequence for a single penetrator of a perforation module in accordance with one embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure relates to devices and methods for perforating wellbores. The present disclosure is susceptible to embodiments of different forms. These are shown in the drawings, and herein will be described in detail, specific embodiments of the present disclosure with the understanding that the present disclosure is to be considered an exemplification of the principles of the disclosure, and is not intended to limit the disclosure to that illustrated and described herein.

FIG. 1 illustrates an exemplary perforation system 10 that is configured in accordance with one embodiment of the present disclosure. The perforation system 10 is disposed within a wellbore 12 that has been drilled through the earth 14 and a hydrocarbon-bearing formation 16. Portions of the wellbore 12 are cased by a steel casing 18 that is secured within the open wellbore hole by cement 20.

The hydrocarbon-bearing formation 16 contains two oil-bearing strata 22, 24, which are separated by a layer of water 26. A layer of water 28 also separates the lower oil stratum 24 from a stratum of gas 30. It is noted that this arrangement of strata in formation 16 is presented only by way of example and that those skilled in the art will recognize that the actual composition and configuration of formations varies.

The perforation system 10 is disposed into the wellbore 12 on a conveyance string 32. The conveyance string 32 may be of any known construction for conveying a tool into a wellbore, including a drill pipe, wireline, production tubing, coiled tubing, and the like. The perforation system 10 includes one or more perforating modules that are used to perforate portions of the surrounding formation 16. In the described embodiment, there are three perforating modules 34, 36, 38 that are secured to one another in series. There may, of course, be more or fewer than three modules, depending upon the desired length of wellbore to be perforated. Additionally, it is pointed out that there may be intermediate sections of tubing, or subs 37 (see FIG. 1 a) interposed between the individual modules 34, 36, and 38, to provide a desired spacing therebetween. In practice, the subs 37 are desirably non-explosive. If desired, the modules 34, 36, and 38 may alternatively be secured to one another so as to form an unbroken, contiguous series of modules. FIG. 1 b illustrates a further alternative perforation system arrangement wherein the perforation modules 34, 36, and 38, of the system are interconnected directly to one another in series.

An exemplary individual module 40 is depicted in FIGS. 2 and 3. The module 40 is representative of each of the three modules 34, 36, and 38 shown in FIG. 1. As will be described in further detail below, the module 40 creates a plurality of perforating penetrators from a single explosive charge. The penetrators travel in a direction substantially normal or orthogonal to the longitudinal axis of the wellbore. Advantageously, this arrangement may significantly increase shot density and simultaneously reduce the amount of debris left in the wellbore, relative to many conventional perforation systems. In one embodiment, the module 40 includes a support member such as a central rod 42 having upper and lower axial ends 44, 46. The upper and lower axial ends 44, 46 are provided with threaded connections, as is known in the art, so that they may be secured to the conveying string 32 (see FIG. 1 b) or to an adjoining module. The central rod 42 is composed of a central load bearing portion 41 and an outer detonation layer 43. The load-bearing portion 41 of the central rod 42 may be a section of pipe, rod or other load bearing structure. In one embodiment, the load-bearing portion 41 of the central rod 42 is formed of steel. In another embodiment, if the perforation device 10 is not to be withdrawn from the wellbore 12 after detonation, the load-bearing portion 41 of the central rod 42 is formed of a frangible or combustible material that will be readily destroyed during the detonation of the perforating device 10. Ceramic is just one example of a suitable frangible material.

The detonation layer 43 comprises, in this embodiment, a primasheet of a type known in the art for initiation of detonations. The load-bearing portion 41 of the central rod 42 may also contain an axial passage 48 along its length to contain electrical wiring 49 that is necessary for initiation of the detonation layer 43 which, in turn, results in detonation of the body 50 of high explosive material. The detonation layer 43 may be initiated with a control signal either manually or utilizing some preprogrammed device. For example, suitable initiating systems can include using electrical signals transmitted from the surface via wiring (not shown) in the axial passage 48 to initiate detonation by increasing hydraulic pressure in the wellbore, or by the dropping of a drop bar (not shown) into the axial passage 48, as is used conventionally with tubing conveyed perforation guns. Other initiating systems can utilize timers or well bore parameter sensitive devices (e.g., pressure, temperature, depth, etc.). Initiation systems for detonating perforating guns are known in the art and will not be discussed in further detail.

Surrounding the central rod 42 is a substantially unitary body 50 of high explosive material that explosively forms the perforating penetrators using the liner 52. Suitable high explosive materials may include, for example, conventionally-employed high explosives such as RDX, HMX and HNS. While the size of the module is not a critical aspect thereof, it may be convenient to configure the module 40 such that it is a cylinder about 12 inches in length and about 4.5 inches in diameter. However, the length and diameter may be varied according to the dimensions of the wellbore 12 or other factors. In one embodiment, a tube 51 of cardboard or a similar material is disposed between the central rod 42 and the high explosive body 50.

The liner 52 surrounds the body 50 of high explosive and is configured to form a plurality of perforating penetrators. The penetrators formed by the liner 52 may travel in a direction generally perpendicular to the longitudinal axis of the wellbore, although modifications in direction may also be achieved in other embodiments of this disclosure. In one embodiment, the liner 52 may be, in this embodiment, a cylindrical and non-explosive liner formed of a metal, such as, for example, tantalum. Alternatively, the liner 52 may be made from extruded copper, tungsten, steel, depleted uranium, aluminum, or another elemental metal or alloy. In other embodiments blends of elemental metals or alloys with materials such as lead, graphite, and zinc stearate may also be employed. In still other embodiments blends or alloys of aluminum with either titanium or hafnium may be used. Additionally, a frangible material may be used to form the liner 52 in order to further reduce the likelihood that the formed penetrator will plug the perforation created in the surrounding formation. Such may include, for example, the use of pressed, sintered metallic powders, such as those described in U.S. Pat. No. 6,012,392, which is incorporated herein by reference in its entirety, and metal/matrix composites.

The size, shape, velocity and other characteristics of the perforating penetrators formed by the liner 52 may be controlled, in part, by adjusting the surface contours and/or geometry of the liner 52. In one embodiment, a plurality of linearly contiguous indentations 54 is formed into the liner 52. As used herein, the phrase “linearly contiguous” means that the perimeters of every indentation shares at least one common side with an adjacent indentation. In some embodiments a majority of each indentation is linearly contiguous with adjacent indentations, and in other embodiments essentially all of each indentation is linearly contiguous with adjacent indentations. In one embodiment, each indentation 54 has an axis that is substantially perpendicular to the exterior surface of the liner 52, where such exterior surface is substantially parallel to the longitudinal axis of the wellbore. In other embodiments such indentation axis may be significantly greater or less than ninety degrees to the exterior surface of the liner 52 and/or to the longitudinal axis of the wellbore, in order to direct the penetrators in a specific direction, according to the purposes and goals of the perforation operation.

FIG. 4 depicts further details concerning one embodiment of the indentations 54. In this embodiment, each indentation 54 has a hexagonal outer perimeter 56 and therefore adjoins a neighboring indentation 54 on each of its six sides, i.e., all of its six sides are linearly contiguous with neighboring indentations 54. Because of this fact, there are no “dead spaces” between the indentations 54 from which it is inferable that there is no area from which a penetrator is not, or could not be, transformed. A small linear ridge 58 is formed at each of the adjoining contact areas of the neighboring indentations 54. A hexagonal shape for the perimeter 56 of the indentations 54 is one possible arrangement, which may offer the additional benefit that, by approximating the shape of a circle, a penetrator that is relatively radially uniform is, upon detonation of the body 50 of high explosive, developed therefrom. Additionally, the hexagonal shape of the perimeter 56 permits relatively closer packing of the indentations 54 to form an adjoining, interlocking honeycomb effect. As a result, the “dead space,” that is unavoidable when indentations having circular perimeters are employed, is thereby greatly reduced or eliminated. A further advantage of the honeycomb arrangement of the indentations 54 is that the perforations created may, as a result, be spaced equally in all directions, that is, in circumferential, axial, vertical, and horizontal directions, such as to significantly reduce the possibility of failure of the surrounding casing 18 upon perforation. A high density of perforations may therefore be achieved from the use of such linearly contiguous and interlocking indentations that cover essentially the entire outer surface area of the module 40. For example, a pattern of hexagonal indentations that are two inches in diameter, i.e., hexagons that can be inscribed within a two-inch diameter circle, may in some embodiments generate a shot pattern of 51 perforations per linear foot of the wellbore from the surface of a 4.5-inch diameter module 40. In contrast, a similarly sized, conventional carrier-type perforating gun, using conventional shaped charges, will typically provide only about 18 perforations per linear foot. Thus, this embodiment illustrates a capability to increase the perforated area by a factor of three. The size and number of hexagonal indentations 54 may be varied, depending upon factors such as the diameter of the module 40 relative to the size of the annular space between the perforation system 10 and the casing wall 18; the properties of the formation in which the perforation gun is being used; the presence or absence of fluid in the annular space; the selection of liner material and explosive; and the like. With the benefit of the present teachings, those skilled in the art will be able to determine optimal configurations based upon such skill and without undue experimentation.

FIGS. 4, 5 and 6 show additional possible configurations for the liner to enable formation of effective penetrators therefrom. As illustrated therein, the indentations 54 each define a cavity 60. While the perimeter of the indentations may influence the shape of the cavity 60, it is not necessarily determinative thereof. Thus, in certain embodiments the shape of the cavity 60 may be of a generally conical or pyramidal configuration, as shown in FIG. 4, or of a generally spherical or parabolic configuration, as depicted in FIGS. 5 and 6. The cavity 60 provides a formation distance for a penetrator to form. The cavity 60 provides an apex 62, i.e., point of greatest indentation, opposite the opening defined by perimeter 56. In this embodiment, the cavity 60 has six equal planar triangular sides 70. The sides 70 adjoin one another along junction lines 72, forming a cavity 60 that is symmetrical along certain axes. The indentations 54 may be formed into the essentially planar liner 52 by stamping, forging or by other known means. Thereafter, the sheet may be formed into a cylinder by bringing opposing ends together and then welding or otherwise connecting the ends. The high explosive body 50 may then be cast into the space between the liner 52 and the inner cardboard tube 51.

The shape of the cavity 60 may determine one or more characteristics of the penetrator and may dictate how the penetrator is formed.

Certain shapes (such as those for conventional shaped charges) produce a Munroe effect, whereby a small fraction (10-15%) of the liner 52 is propelled into a target to cause a narrow and deep penetration. Under the Munroe effect, the tip of the jet formed by a shaped charge liner collapse travels at tremendous velocity (8-10 km/sec). A generally conical or pyramidal cavity 60 produces a jet that forms such a penetration. For the purposes of this disclosure, a Munroe effect penetration may be defined as a penetration wherein a depth of penetration into the formation, i.e., a distance from a cement face, is generally six times or more of the diameter D of the cavity 60.

Certain other shapes produce Misznay-Schardin effect, whereby a large fraction (90-100%) of the liner 52 is propelled into a target to cause a wide and shallow perforation. Projectiles formed under the Misznay-Schardin effect are commonly called Explosively Formed Penetrators (EFPs). EFPs travel much more slowly (·1 km/sec.) than the jet of a conventional shaped charge. A generally spherical, shallow curved hollow, a shallow pyramid indentation, or a shallow concave arcuate shaped cavity 60 forms a projectile that makes such a penetration. For the purposes of this disclosure, “shallow” means that the cavity 60 has a diameter D to depth De ratio of greater than two to one. In some embodiments, the diameter to depth ratio may be six to one or greater. The diameter of the cavity 60 may be measured across the outer perimeter 56 (i.e. diameter D), and the depth De of cavity 60 may be measured from the apex 62 and the plane of the ridges 58. In embodiments where the bottom of the cavity 60 is flattened, then the depth De may be measured between the plane of the flattened area and the plane of the ridges 58. For a non-circular shape, the diameter D may be considered the diameter of the circle that encompasses or circumscribes that shape. In other aspects, the term “shallow” may include designs wherein the depth De of the cavity 60 is approximately the same as or less than a thickness of the liner 52. For the purposes of this disclosure, a Misznay-Schardin effect penetration may be defined as a penetration wherein a depth of penetration into the formation is about one-half to one diameter of the cavity 60. In certain embodiments, the penetration may be less than one-half times the diameter of the cavity 60.

Embodiments of this disclosure contemplate the use of a variety of cavity 60 shapes as required to accomplish the desired penetration.

An alternative method for forming the high explosive body 50 is by pressing a billet to a desired length and diameter, and then machining the billet to match the hexagonal indentations 54 at the outer surface of the liner 52. A long axial hole is then drilled into the center of the billet and sized to accommodate the tube 51. As those skilled in the art are aware, a billet of high explosive is a mass of high explosive material that has been pressed or cast into cylindrical shape. Pressed billets can be machined to a desired shape, while cast billets are formed to the desired shape, such as, in this case, a cylinder with an axial passage therethrough.

FIG. 5 illustrates an alternative design for the indentations 54, here designated 54′. The indentations 54′ still have a hexagonal perimeter 56. However, the side surfaces defining the cavity 60 are smooth and rounded. In side cross-section, the cavity 60 forms a dome-like cap or parabola, as FIGS. 5 and 6, respectively, depict. The radius and apex of each dome-like cavity 60 depend upon the liner thickness and desired formation distance, with the goal that a penetrator may be transformed therefrom that is optimal for creating a large perforation in the wellbore casing 18. In alternative embodiments, other cavity shapes, such as a conical shape, may be employed.

Circumferentially surrounding the liner 52 is a cover 64 that protects the liner 52 and other parts of the module 40 from the harsh wellbore environment. In one embodiment, the cover 64 is a generally cylindrical construction having planar inner and outer surfaces. The cover 64 may be formed of, for example, a thermoplastic or thermoset polymer that is resistant to high wellbore temperatures. The cover 64 may be relatively thin, having a thickness of, for example, just 0.05 inch, and light in weight, such that it will not unduly interfere with the creation of the penetrators from the indentations 54 or 54′. In some embodiments, an elemental metal or alloy, composite material, thermoplastic or thermoset polymer, or glass, for example, may be used to form the cover 64. The cover 64 overlies the adjoining ridges 58 between neighboring indentations 54 or 54′ (see FIG. 6). There is a space disposed between the cover 64 and the ridges 58 to permit the indentations 54, 54′ to fully develop into penetrators upon detonation. Such space may be relatively small, for example, about 5 mm. Air, at atmospheric pressure, may be trapped within the cavities 60 of the indentations 54, 54′ between the cover 64 and the outer surface of the liner 52. The distance between the apex 62 of each indentation 54 or 54′ and the outer cover 64 provides a stand-off for each indentation 54 or 54′ such that a penetrator can more fully develop prior to contact with the well casing 18 (see FIG. 1).

Upper and lower end caps 66, 68 (see FIG. 3) are secured to the cover 64 and liner 52 of the module 40 and serve to help encapsulate and protect the contents of the module 40, particularly the explosive body 50, from fluids within the wellbore 12 prior to detonation.

In operation, the perforation system 10 is lowered into the wellbore 12 until the modules 34, 36, 38 of the perforation system 10 are aligned with the desired strata 22, 24, and 30, respectively, of the formation 14. The modules 34, 36, 38 of the perforation system 10 are then detonated to create penetrators that perforate the casing 18, cement 20 and formation 14. Following perforation of the formation 14, the remains of the perforation system 10 may be removed from the wellbore 12 by pulling upwardly on the conveyance string 32. It is anticipated that, in many embodiments, the perforation modules 34, 36, 38 will be substantially or totally consumed in the detonation.

During detonation of the perforation modules 34, 36, 38, directional penetrators are formed by the indentations 54, 54′. It is noted, however, that the detonation sequence of each module 34, 36, 38, begins at the top end proximate to the central rod 42 and proceeds simultaneously in axially downward and radially outward directions. Each liner indentation 54, 54′, when acted upon by the advancing detonation wave, forms a robust EFP, which is particularly well suited for making large and shallow perforation holes in sandy or soft formations. While conventional shaped charges form a relatively fast-moving, low mass jet that accomplishes the perforation, followed by a relatively slow-moving slug that thereafter carries the mass of the remaining charge liner but does not take part in the actual perforation, the EFP of the present disclosure carries essentially all of the mass of the liner 52 forming the indentation 54 or 54′. This means that the liner mass effectively forms part of the penetrator and takes an active part in the perforation, increasing the relative effectiveness thereof. In one embodiment it has been found that the perforations that result from indentations 54 or 54′ having hexagonal perimeters very closely approximate those created from indentations having circular perimeters.

FIG. 7 illustrates an exemplary shot pattern that may be formed upon detonation of the perforation module 40 within a section 80 of the wellbore 12. FIG. 7 depicts the sidewall of the wellbore section 80 in cylindrical projection with the upper end of the section 80 depicted as line 82 and the lower end of the section 80 shown as line 84. The illustrated wellbore section 80 has a length (L) of approximately one foot. There are fifty-one (51) perforations 86 disposed within the wellbore section 80, which have been created by penetrators formed from the indentations 54 or 54′ of the perforation module 40. In practice, those skilled in the art frequently desire perforations having diameters, as measured at the inner surface of the well casing, ranging from about 10 to about 22 mm, but larger or smaller perforations may alternatively be obtained by simply varying the size of the indentations. In one embodiment, the fifty-one (51) perforations 86 are arranged in six horizontal rows 88 a, 88 b, 88 c, 88 d, 88 e, and 88 f that alternate between eight and nine perforations 86 each. Adjacent rows 88 of perforations 86 are shown herein as horizontally staggered from one another, such that perforations 86 in one row are located diagonal to, i.e., offset diagonally in relation to, perforations 86 in adjacent rows. For example, referring to FIG. 7, perforation 86 b in row 88 b is located diagonal to penetrations 86 a and 86 c in row 88 a. This staggered pattern may be advantageous in some circumstances. Because the penetrations 86 are more densely concentrated than perforations from conventional shaped charge perforation devices, the staggered arrangement may help to avoid overlapping of adjacent perforations. This may be desirable because, if there were numerous such overlaps, the resultant effect of a linear cut in the casing 18 could theoretically produce a casing failure, such as a casing collapse. The staggered arrangement may therefore avoid such an undesirable event. In another embodiment, some of the indentations may be configured of a material that does not suitably form penetrators, in order to reduce the number of penetrators and, therefore, the number or density of perforations obtained thereby. Such an embodiment may be acceptable in certain applications, wherein relatively increased amounts of post-detonation debris are not problematic.

Alternative to indentations having hexagonal perimeters, other perimeter shapes may be selected, most commonly polygonal shapes, such that the perimeters may be adjoined in a linearly contiguous fashion. For example, the indentations may be configured to have triangular, square, or octagonal perimeters. FIGS. 8 and 9 illustrate alternative embodiments wherein such triangular and square perimeter indentations, respectively, are used. FIG. 8 depicts an exemplary perforation module 90 having triangular perimeter indentations 92. As may be seen, the triangular perimeter indentations are located in an adjacent manner such that each of the three sides of a given perimeter borders a side of a neighboring perimeter. Thus, “dead space” between the indentations 92 has thereby been eliminated.

FIG. 9 depicts an exemplary perforation module 94 having square perimeter indentations 96. These indentations 96 are arranged in several horizontally-disposed rows, e.g., 98 a, 98 b, 98 c. Adjacent rows of indentations 96 are staggered relative to one another, i.e., offset by half a square, such that indentations 96 in each row are located with their apices diagonal to the apices of indentations 96 in the adjacent row.

It will be understood by those in the art that each perimeter shape will impart some effect on the configuration of the cavity formed by an indentation, and therefore of the penetrator that will be formed from collapse of the cavity as a result of detonation. Factors such as the fabrication method, and capabilities and limitations thereof, of the liner wherein the indentations are formed, and the material of which the liner is composed, will desirably be taken into account when selecting the perimeter shape and associated packing parameters. For example, triangular and square perimeter indentations may, because of their shape, not collapse as readily during detonation as do hexagonal perimeter indentations in a perforation module wherein all materials and detonation factors are the same. However, modification of such factors may, in some embodiments, offset such disadvantages or even turn such a tendency into an advantage.

FIG. 10 depicts a portion of an exemplary liner surface for a perforation module wherein octagonal perimeter indentations are used. As may be seen in FIG. 10, octagonal perimeter indentations cannot completely cover a given area without leaving some “dead space” between the indentations. In this aspect, their use may be less advantageous, in some embodiments, than the use of hexagonal, square or triangular-shaped indentations. However, octagonal perimeter indentations may more readily approximate the collapse sequence and penetrator transformation of indentations having a circular perimeter, and thus may obtain an advantage over triangular and circular perimeter indentations in certain embodiments. FIG. 10 depicts a liner surface section 100 having a plurality of octagonal perimeter indentations 102 that adjoin, i.e., are linearly contiguous to, one another at four of their eight sides 104. The remaining four sides 106 of the octagonal perimeter indentations 102 define square areas 108 as interstitial spaces. If desired, the interstitial square areas 108 may themselves be indented, in the manner of square indentations 96 (see FIG. 9), to provide for additional formed penetrators.

Turning now to FIGS. 11 through 15, an exemplary initiation sequence is illustrated for a single EFP from a perforation module 40. FIG. 11 is a cross-sectional view of the indentation 54 prior to detonation of the perforation module 40. The indentation 54 is formed in liner 52 that surrounds the high explosive body 50. In this embodiment a thermoplastic cover 64 surrounds liner 52. The module 40 is disposed within a section of wellbore casing 18 surrounded by cement 20. Fluid 57 resides in the annular space that is between the casing 18 and the radially exterior portion of the cover 64. FIG. 12 depicts the beginning portion of the detonation wherein the material forming metallic liner 52 has begun to collapse or coalesce within the space formerly occupied by the cavity 60 of indentation 54. The cover 64 atop the indentation 54 has begun to bow outward and thin out. In FIG. 13, the detonation process has progressed to the point where a generally spherical penetrator 110 has been formed from the material making up the liner 52. The casing 18 and fluid 57 are essentially sheared through by the penetrator 110. FIG. 14 depicts an advanced stage of the detonation with the penetrator 110 now in a primarily plastic phase and perforating the cement 20 on its way to the formation (not shown). The walls of the perforation cavity may be lined with the liner or EFP material 25 that originally formed at least part of the liner 52 before detonation. Thus, aspects of the present disclosure include at least partially or substantially lining a perforation in the cement with liner material. Some unspent residual liner or EFP material 27 may be present at the end of the perforation.

Referring now to FIG. 15, the penetrator 110 has formed a tunnel in the cement 20 and has proceeded a distance 21 into a formation 16. The distance 21 may be generally measured from a boundary 23 between the cement 20 and the formation 16. The distance 21 may be approximately a diameter of the cavity 60 or less. In variants, the distance 21 may be one-half or less of the diameter of the cavity 60.

Referring now to FIG. 1, in an illustrative method of perforating a well, the perforation system 10 may be disposed into the wellbore 12 on the conveyance string 32. As noted previously, the perforation system 10 may include one or more perforating modules that are used to perforate portions of the surrounding formation 16. The formation 16 may be loosely consolidated, unconsolidated or un-compacted. In one aspect, such a formation includes sediment, rocks, sand, and other granular material that behave more as a fluid than a solid. In another aspect, the particles making up such a formation may move relative to one another. In still another aspect, such a formation is not sufficiently stable to maintain a cavity or conduit formed therein. For example, a tunnel formed into the formation would collapse soon after formation. In a generally unconsolidated formation 16, it may be desired to perforate the casing and the cement, but leave the formation 16 largely untouched. For such an application, the perforating modules 34, 36, 38 may utilize indentations configured to utilize the Misznay-Schardin effect forming EFPs. There may, of course, be more or fewer than three modules, depending upon the desired length of wellbore to be perforated.

Upon the firing of the perforating modules 34, 36, and 38, perforations will be formed in the casing and cement in a manner as generally shown in FIG. 15. However, the unconsolidated formation adjacent to the casing and cement are largely untouched. In aspects, therefore, the present disclosure provides a method of forming fluid channels in a well to produce fluids from a subsurface hydrocarbon reservoir that resides in an unconsolidated or substantially unconsolidated formation. In embodiments, the perforations in the cement may be at least partially lined with liner material.

In summary of the foregoing description, those skilled in the art will appreciate that the design of the perforation system 10 thus provides a number of advantages over conventional perforation systems. Included among these, first, is the fact that the linearly contiguous packing of the indentations combined with the unitary body of high explosive produces a greater number of perforating penetrators over a given axial length of a module 40 and reduced amount of “dead space,” as compared with conventional perforation systems using shaped charges and indentations that are physically separated and/or have circular perimeters. The greater number of penetrators results in a desirably greater density in the post-detonation perforation shot pattern. Second, the disclosure provides for a substantial reduction in debris formed during the perforation operation. And third, the perforation module 40 may be created or manufactured and customized relatively easily, without the need for time-consuming placement and orientation of individual shaped charges, as with conventional systems.

From the above, it should be appreciated that what has been described includes, in part, a method for perforating a subterranean formation. The method may include lowering a plurality of perforating modules in a wellbore and detonating the plurality of perforating modules in the wellbore. Each of the plurality of perforating modules may include a central rod having an exterior load bearing portion, and an interior detonation portion including a first explosive, a second explosive at least partially surrounding the central rod, and a liner surrounding the second explosive, wherein the liner has a plurality of shallow concave surface indentations. The central rod further may include an axial passage. In one arrangement, a conducting wire may be disposed within the axial passage. In another embodiment, the axial passage may receive a hydraulic fluid. In embodiments, the plurality of shallow concave surface indentations may each have a cavity. The cavity may be defined by a diameter and a depth. In one arrangement, the diameter to depth ratio is approximately not less than two to one. In another embodiment, the diameter to depth ratio is not less than six to one. In still other embodiments, the depth is no greater than a thickness of the liner.

In arrangements, the plurality of shallow concave surface indentations may each have a cavity. The method may include forming a plurality of perforations in a region adjacent to the perforating modules, wherein the plurality of perforations extend substantially through a cement layer into a formation a distance from a cement face that is no greater than a diameter of the cavity. In some applications, the distance is no greater than one-half of the diameter of the cavity. Also, the method may include at least partially lining the plurality of perforations in the cement layer with a liner material.

In embodiments, the exterior load bearing portion may comprise a frangible material. Also, each of the plurality of shallow concave surface indentations may be linearly contiguous with at least another of the plurality of shallow concave surface indentations. In some arrangements, the shallow concave surface indentations may be arcuate.

From the above, it should be appreciated that what has been described also includes, in part, a method for perforating a subterranean formation. The method may include lowering a perforating module into a wellbore having a casing incased in cement, the perforating module having at least one explosively formed penetrator forming charge and liner; positioning the plurality of perforating modules in the wellbore and adjacent to a substantially unconsolidated formation; perforating the casing and cement; and perforating the formation to a distance no greater than a diameter of the at least one explosively formed penetrator forming charge and liner, wherein the distance measured from a boundary between the cement and the formation.

The foregoing description is directed to particular embodiments of the present disclosure for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope of the disclosure. Thus, it is intended that the following claims be interpreted to embrace all such modifications and changes. 

1. An apparatus for perforating a subterranean formation, comprising: a plurality of perforating modules in a wellbore, wherein each of the plurality of perforating modules comprises: a central rod having an exterior load bearing portion and a detonation layer formed of a first explosive exterior to the exterior load bearing portion; a second explosive surrounding the central rod, wherein the second explosive is a unitary body; and a liner surrounding the second explosive, wherein the liner has a plurality of horizontal rows of concave surface indentations, the second explosive explosively forming a plurality of shaped charges from the liner.
 2. The apparatus of claim 1, wherein the liner circumferentially surrounds the second explosive.
 3. The apparatus of claim 1, wherein the detonation layer comprises a layer of primasheet.
 4. The apparatus of claim 1 wherein the central rod connects with a conveyance string.
 5. The apparatus of claim 1, wherein the liner is formed of tantalum.
 6. The apparatus of claim 1, further comprising a substantially cylindrical cover member surrounds the liner.
 7. The apparatus of claim 1, wherein each indentation has a perimeter that is one of: (i) triangular; (ii) square; (iii) pentagonal; (iv) hexagonal; and (v) octagonal.
 8. The apparatus of claim 1, wherein the indentations are arranged in a pattern over outer radial surface of the liner such that each indentation is linearly contiguous with at least one other indentation.
 9. An apparatus for perforating a subterranean formation, comprising: a plurality of perforating modules in a wellbore, wherein each of the plurality of perforating modules comprises: a central rod having an exterior load bearing portion and a detonation layer formed of a first explosive exterior to the exterior load bearing portion; a second explosive surrounding the central rod, wherein the second explosive is a unitary body; and a liner surrounding the second explosive, wherein the liner has a plurality of horizontal rows of concave surface indentations, wherein the indentations are arranged in a pattern over an outer radial surface of the liner such that each indentation is linearly contiguous with at least one other indentation. 